1. Field of the Invention
The present invention relates to microelectromechanical sensors. More particularly, this invention pertains to a microelectromechanical sensor and a method for operating such microelectromechanical sensor.
2. Description of the Prior Art
Microelectromechanical sensors are integral to many technical devices. They have proved to be advantageous, for example, in the field of navigation, where they are employed as Coriolis gyroscopes. (The functioning of a microelectromechanical sensor is explained below with reference to a Coriolis gyroscope.)
Coriolis gyroscopes include a mass system that can be caused to effect oscillations. The mass system generally possesses a multiplicity of oscillation modes that are initially independent of one another. In the operating state (i.e. in the operating state of the microelectromechanical sensor), a specific oscillation mode of the mass system is excited artificially (“excitation oscillation”). When the Coriolis gyroscope is rotated, Coriolis forces occur that draw energy from the excitation oscillation of the mass system and transfer it to a further oscillation mode of the mass system (“read-out oscillation”). In order to determine rotations of the Coriolis gyroscope, the read-out oscillation is tapped off and a corresponding read-out signal examined for changes in the amplitude of the read-out oscillation which provide a measure of the rotation of the gyroscope. Coriolis gyroscope may comprise either an open-loop or a closed-loop system. In a closed-loop system, the amplitude of the read-out oscillation is continuously reset to a fixed value (preferably zero) by means of associated control loops, and the resetting forces measured.
The mass system (“resonator”) of the Coriolis gyroscope (the microelectromechanical sensor) can be configured in a variety of ways. It is possible to arrange a mass system in one piece or to divide the mass it into two oscillators, coupled to one another via a spring system, that are capable of performing relative movements with respect to one another.
FIG. 1 is a schematic illustration of a Coriolis gyroscope 20 in accordance with the prior art. It includes a charge amplifier 1, an analog-to-digital converter 2, a signal separation 3, demodulators 4, 5, a control system 6, a modulator 7, drivers 8, 9, a resonator 10 and an electrode system 11 having four electrodes 111 to 114. The resonator 10 can be excited by means of the electrodes 113 to 114 to cause oscillations. Furthermore, the spring constant of the resonator 10 can be electrostatically set or changed by the electrodes 111 to 114. Movement of the resonator 10 is determined by measuring a charge transfer Δq on an electrode provided on the resonator 10 (“movable center electrode”), which is caused by movement of the resonator 10 within the electrostatic field generated by the electrodes 111 to 114. A signal S7, proportional to the charge transfer, is output by the charge amplifier 1 to the analog-to-digital converter 2 and converted by the latter into a corresponding digital signal S8 that is fed to the signal separation 3. Signals S3 to S6 are generated from this signal with the aid of the demodulators 4, 5, the control system 6 and the modulator 7 and the drivers 8, 9. Such signals are applied to the electrodes 111 to 114 so that deflections of the resonator 10 caused by Coriolis forces are compensated. Details of the operation of the Coriolis gyroscope 20, are provided, for example, in German patent specification DE 103 20 675.